The present invention relates to an improvement in a furnace or similar apparatus for thermal treatment of semiconductors such as wafers for the production of integrated-circuit chips.
As a consequence of recent trends, there has been a gradual tendency for semiconductors to be processed as wafers of larger sizes, but various problems thus have arisen. As an example, convection currents of a treating gas become significant, and it thus becomes difficult to maintain uniform temperature distribution within a furnace for thermal treatment of semiconductors.
In such apparatus, a wafer placed on a boat is set substantially at the center of a treating tube, and a treating gas is introduced into the treating tube from a gas inlet. A predetermined treatment is carried out while the treatment temperature is maintained at a prescribed level, e.g., 400.degree. to 1200.degree. C.
In such thermal treatment, it is required that the internal temperature be uniform axially of a wafer or the like to be treated and, moreover, to be uniform in a direction perpendicular to the axis of the treatment tube. Ordinarily, it is desired that the temperature fluctuation be controlled within a range of .+-.1.degree. C.
In prior efforts to achieve a uniform temperature within a typical furnace for thermal treatment of semiconductors, it is known for the length of the furnace to be increased or for a heater associated with the furnace to be divided. Typically, the heater is divided axially into three parts, and electric power applied to the heater is set so that the part of the heater at each end provides temperatures higher than the temperature provided by the part of the heater therebetween, whereby the length of a region of uniform heating temperature extending longitudinally of the treating tube is increased.
A boat carrying a wafer is inserted into and withdrawn from the treating tube after a removable closure, which may be attached to an end portion of the treating tube, has been detached therefrom. Therefore, so as to avoid rapid heating of a wafer or the like to be treated or rapid cooling thereof, an extra space must be provided in the treating tube for moderating thermal influences on the wafer or the like. Ordinarily, the end portion of the treating tube on the closure end, which is exposed to the outer atmosphere, is elongated to form a low-temperature zone having a considerable length.
However, the extra space, when provided, is cooled by contact of the elongated portion of the treating tube with the outer atmosphere so as to lower the temperature within the elongated portion of the treating tube, whereupon the length of the region of uniform heating temperature is reduced. Furthermore, convection currents are produced between the extra space and the zone of uniform heating temperature extending longitudinally of the treating tube, and such currents circulate concentrically in the upper and lower portions of the region of uniform heating temperature. Thus, a low-temperature zone is formed, which divides the region of uniform heating temperature into two parts, and effective length of the region of uniform heating temperature is reduced.